1. Field of the Invention
The present invention pertains to the polishing of commercially important articles, such as hard disk blanks, and in particular to the polishing of articles using free abrasive machining techniques.
2. Description of the Related Art
The polishing of thin, flat objects plays an important part in many commercial applications. For example, hard disk blanks are machined using free abrasive processes to flatten one or both major surfaces of the disk. Such flattening is carried out to a high degree of accuracy, so as to produce what is commonly termed a "mirror surface" or an "optically flat" surface. One example of a family of such machines is offered for sale by the Assignee of the present invention under the Model designation SFDSM. With these machines, both sides of a workpiece may be processed at the same time to achieve desired surface polishing.
In typical commercial scale operations, several workpieces are polished on one machine at one time. For example, in one type of polishing machine both major surfaces of the workpieces (e.g., memory disk blanks or disk substrates) are simultaneously polished using a planetary motion. In such "double sided" machines, provision is usually made to move the upper polish plate, workpieces (i.e., planetary carrier system), and lower polish plate independently of one another so as to achieve various, desired polishing results. In some machines of this type, the upper polish plate is conveniently driven from above. However, in other types of machines such as those addressed by the present invention, the upper polish plate is driven from below, using a drive hub located along the center axis of the polishing machine. Such arrangements have arisen, in part, since the upper and lower polishing plates typically have an annular configuration with central openings of substantial size. It is possible, therefore, to provide a plurality of concentric, nested drive arrangements in the base of the polishing machine in such a way that the drive systems do not interfere with one another and can be operated independently.
It is known to provide rotational drive for the upper plate using a slotted, upwardly extending hub. The slots extend in a vertical direction, the direction of travel of the upper plate as the plate is lowered onto the workpieces. Blade-like drive latches are pivotally mounted on the top side of the upper plate and have inner edges received in the slots so as to be carried for rotation with the drive hub. During a polishing operation, radial and other lateral forces are developed between the upper polish plate and the drive hub, causing the latches to shift within the slots, resulting in a changing depth of penetration of the latches within the slots. It is desirable, in general, to eliminate or reduce such lateral excursions, especially when such excursions are observed in members closely related to the polishing process.
Loading and unloading of the workpieces is typically accomplished by separating the upper and lower polish plates by a distance sufficient to allow a machine operator or an automated load/unload device to grasp and remove the workpieces, thereafter replacing the polished workpieces with fresh workpieces in preparation for a subsequent cycle of operation.
There have been constant ongoing demands within the industry to achieve workpiece surfaces of ever increasing flatness. Considerable effort and expense have been incurred and substantial improvements in surface flatness have been attained. However, end users of disk substrate workpieces require flatter surface finishes, in part, to improve hard disk storage densities and increasing data throughput rates.